The present invention relates to a process for manufacturing a flexible pipe which can be used for transporting fluids such as, for example, hydrocarbons.
Several types of flexible pipes are used. Some flexible pipes comprise, from the inside to the outside, an internal sealing sheath or plastic, elastomer or another relatively pliant suitable material; a non-sealed flexible metal tube which must withstand the loads developed by the pressure of the fluid flowing in the pipe; one or more armouring plies and at least one external sealing sheath of polymeric material. This type of flexible pipe is often called a xe2x80x9csmooth-borexe2x80x9d flexible pipe by those skilled in the art.
Other flexible pipes called xe2x80x9crough-borexe2x80x9d flexible pipes comprise, from the inside to the outside, a non-sealed flexible metal tube, called a carcass, consisting of a section wound with turns which are mutually interlocked, such as, for example, an interlocked strip or an interlocked shaped wire such as a T-shaped, U-shaped, S-shaped or Zeta-shaped wire, an internal sealing sheath of polymeric material, one or more armouring plies capable of withstanding the loads developed by the pressure of the fluid flowing in the pipe and of withstanding the external loads to which the flexible pipe is subjected, and at least one external protective sheath of the polymeric type.
In the latter type of flexible pipe, the internal sealing sheath is continuously extruded directly onto the said carcass which has interstices or separations between the wound turns.
In order to ensure good contact between the internal sealing sheath and the metal carcass, it is necessary for the internal diameter of the internal sealing sheath to be as close as possible and even equal to the external diameter of the flexible metal carcass.
During manufacture of a flexible pipe of the rough-bore type, the internal sealing sheath, which is extruded onto the metal carcass, contracts on the latter during cooling. Depending on the materials used for producing the internal sealing sheath, deformations called xe2x80x9ccontraction cavitiesxe2x80x9d are observed, after cooling, these contraction cavities appearing on the internal surface of the said internal sealing sheath and, in particular, on either side of the separations between the turns of the metal carcass. Such contraction cavities are due, it would seem, to the differential shrinkage of the material used for the internal sealing sheath because of the variation in the cooling gradient in the thickness of the internal sealing sheath, combined with the effect of the separations of the turns of the metal carcass. This is because, since the extruded sealing plastic sheath is in contact by its internal surface with the metal carcass which is at ambient temperature, the resulting cooling of the said internal surface is very rapid, thereby causing surface irregularities or contraction cavities. This phenomenon is amplified in the separations of the turns of the metal carcass, the differential shrinkage at these points causing local thickness variations in the internal sealing sheath. When the sealing sheath is made of a semicrystalline polymer which is sensitive to the presence of surface defects leading to a failure of the sheath possibly resulting in rupture, such as, for example, PVDF, this very often leads, in operation, to degradation of the said sealing sheath (rupture) which then no longer fulfils its sealing function.
In order to remedy such a drawback and to solve the problem posed by the appearance of contraction cavities, the solution consisting in arranging, between the metal carcass and the internal sealing sheath, a thin sacrificial sublayer of a suitable material, such as PVDF, was discovered and adopted. The internal sealing sheath is then extruded onto the said sacrificial sublayer, but ensuring that there is no intimate bonding or xe2x80x9cweldingxe2x80x9d between the sealing sheath and the sacrificial sublayer so that cracks propagating from the internal surface of the sublayer towards the outside are blocked at the interface of the sealing sheath and the sacrificial sublayer.
The major drawback of this solution is the slip likely to occur between the internal sealing sheath and the sacrificial sublayer near the ends of the flexible pipe, as well as the additional raw-material and conversion costs incurred by the presence of the said sacrificial sublayer.
Other solutions for eliminating the appearance of contraction cavities, or for reducing their effects, have been sought.
The latter solutions, the subject of which is the installation of an internal sealing sheath having, after cooling, a smooth cylindrical internal surface, involve a conformation which is either internal, with as main drawback the creation of longitudinal cracks on the internal surface of the sealing sheath and a fold of material on the external surface, or which is external, with as drawback the complete absence of anchoring of the sealing sheath to the metal carcass.
In the technique of manufacturing flexible pipes of the smooth-bore type, which consists in producing separately the internal sealing sheath, by any suitable means such as extrusion, and the metal carcass, it has been recommended to heat the sealing sheath or the metal carcass once the two elements have been joined together, so that the sealing sheath is rendered or maintained in a plastic form in order to force it to flow plastically into the separations of the turns of the metal carcass. Such manufacturing processes are described in particular in FR-A-74 14 398 (Coflexip) and the addition No. 71 16 880 (IFP).
However, the sole object of these processes is to cause permanent plastic flow of the polymeric sealing sheath between the turns of the metal carcass, after or at the same time as stresses are developed in the internal sealing sheath in order to achieve intimate contact, the stresses developed being, for example, due to compression of the said internal sealing sheath.
In an embodiment described in the patent FR-A-74 14 398, relating to a flexible pipe comprising a peripheral sheath extruded onto an assembly comprising, from the inside to the outside, an internal sealing sheath, a pressure vault, two armouring plies and a metal trellis, it is recommended to heat the assembly before extruding the peripheral sheath so as to maintain at least the internal surface of the said peripheral sheath in the plastic state or more precisely in the thermoplastic state so as, and it is this which is the desired object, to cause the internal surface to flow plastically into the meshes of the metal trellis in order to fill them completely and thus achieve complete fastening of the peripheral sheath to the metal trellis. Under these conditions, it is essential to heat the assembly strongly, to temperatures which are of the order of several hundred degrees Celsius. Such techniques have given such poor results that they were abandoned very rapidly since filling both the interstices of the pressure vault and the meshes of the metal trellis stiffened the pipe and consequently diminished the essential property of flexibility which the pipe must necessarily have.
The object of the present invention is to propose a process for manufacturing flexible pipes of the rough-bore type, which makes it possible to eliminate the contraction cavities or other surface irregularities liable to appear on the internal surface of the internal sealing sheath.
The subject of the invention is a process for manufacturing flexible pipes, which is characterized in that it consists in heating the flexible metal tube or metal carcass to a temperature below 100xc2x0 C., upstream of the extrusion means, so as in this way to avoid sudden cooling of the internal surface during extrusion onto the metal carcass.
Another characteristic of the present invention resides in the fact that the process for manufacturing a flexible pipe of the type comprising, from the inside to the outside, a flexible metal tube having a helical winding of turns, an internal sealing sheath arranged around the said flexible metal tube, at least one armouring ply wound around the said internal sealing sheath and at least one external sealing sheath, the internal sealing sheath being extruded onto the flexible metal tube using extrusion means, characterized in that it furthermore consists in heating the said flexible metal tube upstream of the extrusion means and at a temperature below 100xc2x0 C.
The process is characterized in that the temperature to which the flexible metal tube is heated is less than the temperature for which the material of the internal sealing sheath would fill, by plastic flow, at least 95% of the volume of the spaces between the turns of the said metal carcass.
The process is characterized in that the temperature to which the flexible metal tube is heated is between 20 and 80xc2x0 C.
The process is characterized in that the temperature is more or less constant over the entire periphery of the flexible metal tube.
The process is characterized in that the heating means consist of an infrared heating device or any other suitable means such as, for example, induction heating means, convection heating means, etc.
The process is characterized in that the temperature to which the flexible metal tube is heated is determined depending on the nature and on the extrusion temperature of the constituent material of the internal sealing sheath and on the thermal properties and geometrical characteristics of the said carcass.
The process is characterized in that the temperature to which the flexible metal tube is heated is determined depending on the speed at which the said flexible metal tube runs through the extrusion means.
The process is characterized in that the flexible metal tube consists of an interlocked strip.
The process is characterized in that the flexible metal tube is produced by a helical winding of wires whose cross-section is, in particular, S-, T-, U- or Zeta-shaped.
That being so, the internal sealing sheath flows plastically only partially into the separations, thereby preserving the flexibility of the pipe while at the same time avoiding the creation of contraction cavities which are a source or defects liable to accelerate deterioration of the internal sealing sheath when cracks appear or are liable to appear during use of the flexible pipe according to the invention.
It should be noted that heating the carcass in no way contributes to the actual fastening of the internal sealing sheath to the carcass, this fastening being achieved downstream of the extrusion head by a partial vacuum in the annular space between the carcass and the internal sealing sheath.